This invention relates to a device for separating plasma or serum from whole blood.
In clinical chemistry, the separation of serum or plasma from whole blood is of outstanding importance since, in practice, it is not possible to carry out the analysis of dissolved blood components without disturbances unless the separation takes place.
The normal and most conventional manner of separating serum or plasma from erythrocytes is centrifuging. Centrifugation, however, causes problems with respect to separating supernatant and blood cake. This problem is particularly acute when small amounts are involved. To that end, a whole series of adjuvants have been described in the literature. See, in this regard, Federal Republic of Germany Patent Specification No. 25 59 242 (U.S. Pat. No. 4,012,325).
The use of whole blood in the case of rapid diagnostic agents gives rise to special problems. Rapid diagnostic agents are reagent-containing carriers, which are absorbent or swellable. Preferably, they are made of filter paper. In use, one applies a small amount, such as a droplet, of the liquid to be investigated to the carrier. After a short period of time to allow reactions to take place, a color change occurs, which can be evaluated visually or via remission photometry. Since turbid or coloured solutions, such as blood, disturb the readings, attempts have, therefore, been made to make rapid diagnostics available for the direct use of whole blood. Thus, for example, mention may be made of the coating of test papers with semi-permeable membranes (see U.S. Pat. No. 3,092,465) and the use of swellable films into which only the dissolved components of the blood can penetrate but not the erythrocytes (see Federal Republic of Germany Patent Specification No. 15 98 153; U.S. Pat. No. 3,630,957). These two methods are per se usable but only for tests for low molecular weight components of blood, for example glucose or urea. Higher molecular weight components of the blood, such as lipids, or substrates bound to serum protein, such as bilirubin, cannot be determined in this way because they are not able to penetrate into the film or to pass through the semipermeable membrane. Furthermore, suggestions have been made for covering diagnostic agents with membrane filters for separating off the blood cells (see Federal Republic of Germany Patent Specifications Nos. 22 22 951 and 29 22 958 which correspond to U.S. Pat. Nos. 3,663,374 and 4,256,693). A disadvantage of these diagnostic agents is that the blood can only penetrate through the membrane filter very slowly and in small amounts because the membrane is very easily blocked up. This results in a reaction which takes longer than is desired. In contradistinction to the previously-mentioned diagnostic agents, which are already commercially available, "rapid tests" of the last-mentioned type have not yet been marketed.
Federal Republic of Germany Patent Specification Nos. 29 08 721 and 29 08 722 which correspond to U.S. Pat. Nos. 4,246,107 and 4,330,410, teach that lymphocytes and leukocytes can be separated from blood when blood is filtered through a layer of synthetic resin fibres with an average fibre diameter of 5 to 20.mu. (lymphocytes) and of 3 to 10.mu. (leukocytes). However, since the erythrocytes preponderantly pass through the filter with the plasma, these filters are not suitable for obtaining plasma. Furthermore, carbon fibres, glass fibres and metal fibres are mentioned purely speculatively.